POWER GENERATION AND TRANSMISSION 19
Where low-pressure steam can be used for heating and process work, a simple engine may preferably be run non-condensing and even at considerable back pressure: or a specially designed compound employed and steam taken from the receiver between the cylinders. The Uniflow engine with a single cylinder gives economies (condensing) equal to that of the best ordinary compounds.
While the steam turbine has largely displaced the engine, the latter has still a distinct field. It is to be preferred where speed variation is necessary, where starting conditions are severe and for low-pressure operation of small units using saturated steam, whether condensing or non-condensing.
Size. For simple engines,
JpmAS fpmALN m
Lkp' = 3pOO = -33W = /PmK ( }
~P (2)
In the above,
I.hp. = indicated horsepower to be ex- N = revolutions per minute (r.p.m.) pected from the engine, S = 2LN piston speed, feet per
A = average effective area of the minute,
piston, square inches, K = ALY/16,500 = AŁ/33,000, a
L = stroke of piston, feet, constant for a given engine.
In an ordinary double acting engine, A is the cross-sectional area of the cylinder minus half the cross-sectional area of the piston rod. When a tail rod is used, the deduction is the whole cross-sectional area of the rod. P = pressure at throttle, pounds per square inch, absolute, p = pressure in exhaust pipe, pounds per square inch, absolute, R = ratio of expansion = volume of steam when piston is at outer end of stroke
-=- volume of steam at point of cutoff. and / is the socalled diagram factor, values of which are given below.
Pressures and Expansion Ratios [for Equation (2)]. In ordinary practice, P ranges from 75 to 250. Common values for simple engines are from 95 to 115. For non-condensing engines exhausting to the atmosphere at sea level, p is between 15 and 17. For condensing engines, p is between 1 and 2^. The value 2 should be used in design (Ennis, "Vapors for Heat Engines/7 1912, p. 38). If the exhaust steam from an engine is used for heating buildings or in manufacturing processes, p may have any value from 15 upward.
The value of R is commonly around 4 in simple engines. It should increase as P increases and as p decreases, being usually between 3 and 5. It should be higher in jacketed than in unjacketed engines. The efficiency of the engine depends largely on the value chosen for R. High values of R will be adopted for an engine to be used where fuel is costly or the load steady. The overload capacity is similarly influenced: low values of R lead to high mean effective pressures and hence to large output from a cylinder of given size but also to low overload capacities. In general European practice cut-off is fixed so as to give pm = 1.2 + 0.2P.
Values of /:
Independent cut-off, cylinder jacketed ......................... 0 .90
Single valve, automatic cut-off, cylinder jacketed ................ 0 . 86 to 0 . 88
Single valve, automatic cut-off, without jacket .................. 0 . 77 to 0 . 82
Unjacketed throttling engines of small size and high speed ........ 0.58 to 0.77